Simplified model systems, including cell culture, are being utilized to investigate aspects of nervous system development and regulation of function. Specifically, (1) some characteristics and specificities of the factor(s) in culture medium of muscle cells (conditioned medium) which causes increases in the activity of the enzyme choline acetyltransferase in spinal cord cell cultures, have been elucidated. (2) Cephalic neural crest cells, the cells in the outgrowths from explants of chick embryo cranial neural tube, were found to develop rapidly significant levels of choline acetyltransferase activity in temporal conjunction with morphological events which suggested neuronal development and formation of interneuronal connections. These morphological and biochemical changes could be controlled and modified by changes in the type and serum content of the growth medium. (3) Ablation of the habenular nuclei in the rat brain resulted in a rapid (3 days) and marked (greater than 93%) loss of choline acetyltransferase activity in the interpeduncular nuclei, apparently due to loss of innervation via the habenulo-interpeduncular tracts. After almost complete loss of that activity, there followed a marked increase in the specific activity of glutamic acid decarboxylase in the nuclei. BIBLIOGRAPHIC REFERENCES: Mata, M. M., Schrier, B. K., Klein, D. C., Weller, J. L. and Chiou, C. Y.: On GABA function and physiology in the pineal gland. Brain Res. 118: 383-94, 1976. Rowe, V., Neale, E. A., Avins, L., Guroff, G. and Schrier, B. K.: Pineal gland cells in culture: Morphology, biochemistry, differentiation, and co-culture with sympathetic neurons. Exper. Cell Res. 104: 345-56, 1977.